Physical shipment of materials, including the shipment of mail, merchandise, raw materials, and other goods, is an integral part of any economy. Typically, the materials are shipped in a type of shipping container or cargo box, which are generally transported via semi-trailers, large trucks, and rail cars as well as inter-modal containers that can be carried on container ships or cargo planes. Such shipping or cargo containers, however, are also sometimes used for illegal transportation of contraband. Detection of these threats requires a rapid, safe and accurate inspection system.
High-energy x-ray inspection is employed worldwide to detect contraband including drugs, currency, weapons and manifest violations. Contraband detection is typically performed by analyzing images for anomalies. Oftentimes when anomalies are identified as potential contraband, manual labor intensive and time-consuming unpacking is required. In some cases, equipment has to be damaged to determine whether contraband is actually present. Unfortunately, these anomalies may be produced by the natural variation of benign cargo, therefore leading to a false alarm situation where the manual inspection was unnecessary.
Known scanning processes for inspection of containerized cargo include X-ray scanning, chemical analysis of vapour emitting from the cargo, listening to sound from the cargo to detect living objects and eventually interventional manual search of the cargo by one or more security officials. In some systems, neutrons are employed in secondary inspection techniques and methods to detect and/or clear the presence of explosives and other materials. For example, Rapiscan Systems, Inc. has a Vehicle Explosive Detection System (VEDS) that employs a moderated 232Cf spontaneous fission source or an Electronic Neutron Generator (ENG) such as d-D or d-T to produce neutrons for inspecting a cargo container. In most cases, the neutrons are mainly uncollimated, impinge upon a large area of the container, are not depth sensitive and provide limited elemental information. Therefore, these systems can detect medium amounts of contraband and are limited to some types of materials.
One of the advantages of employing neutrons is that their interaction with matter results in gamma rays. These gamma rays are characteristic of the elements that produced them and therefore, can be used to deduce the elemental composition. When an object is interrogated with neutrons, gamma-ray signals are produced from different parts of the object; signal mixing is reduced by determining the position of mono-energetic neutrons as a function of time. This, in turn, yields gamma ray information as a function of time. As the speed of neutrons is known, the location of where the gamma rays were produced can be computed. This provides a determination of the elemental composition of the scanned area as a function of depth, with little mixing of signal from other areas.
With continuous wave (CW) sources—which produce radiation continuously, or pulsed at micro-second intervals, it is very difficult to determine the location where gamma rays are produced. There is a superposition from gamma rays produced in the front, the center and the back of the object and the deduced elemental composition is mixed. For example, if there is an amount of cocaine located in the center of a paper-loaded container, the gamma rays will present mainly from the paper and minimally from the cocaine, in which case, the cocaine may remain undetected. This is because the neutrons will interact more at the front than at the center due to attenuation. Since there is no time (depth) information, the elemental signal from cocaine (very little signal) is summed up with the elemental signal of paper (more abundant signal). For example, the signal of cocaine is C=4 and O=1 (C/O=4). The signal of paper is C=10 and O=10 (C/O=1). The measured signal is C=14 and O=11 with a C/O of 1.3. However, if time (depth) information is present, information from the front, center, and back is separated into discrete, detectable signals. Thus, TNA (Thermal Neutron Analysis) does not work well because of the mixing of signals as a function of depth.
By way of example, in Pulsed Fast Neutron Analysis (PFNA) technology, a high-energy pulsed deuteron beam impinges on a deuterium target to produce an intense nano-second pulsed neutron beam, which allows for determination of the elemental content of the area being inspected. The cross section (x-y) mapping is obtained by the use of collimation and the depth (z) map is obtained using time-of-flight (ToF) technology. PFNA can be used for primary inspection and/or for secondary inspection. In the secondary approach, a primary system (PFNA, x-ray or other) identifies areas suspected of containing contraband which are then inspected with a collimated neutron beam. Although PFNA is a very powerful technology, a system based on this technology is large and expensive, which limits its deployment.
Similarly, Associated Alpha-Particle Imaging (API) employs a partially collimated neutron beam to inspect an object, whereby an elemental map of the object can be determined. The cross-sectional elemental map is obtained by detecting the associated alpha particle direction, which is emitted 180 degrees relative to the direction of the emitted neutron. The depth map is also obtained using ToF technology but instead of using a pulsed-neutron beam, the detection of the alpha particle provides the starting time. When deuterons from the generator hit the tritium target, the nuclear reaction results in an alpha particle and a neutron, produced 180 degrees from each other. The alpha particle is detected first because the alpha detector is proximate. Thus, it can be used to start the clock to determine where the associated neutron is. If at t=0 the neutron is at 10 cm, at t=1 ns, it will be at 15 cm and at t=2 ns, it will be at 20 cm, because neutrons move at approximately 5 cm/ns.
Due to the random coincidences of the alpha particle and neutron-induced gamma ray measurements, the resultant signals are affected by a high background that limits the maximum neutron output. This requires lowering the neutron output to a level where this background is low, but results in long inspection times, reducing the throughput.
While d-D and d-T neutron generators employing techniques similar to PFNA and API have been used, they have not been widely deployed due to either size and cost limitations, or long inspection times as a result of low neutron yield. A d-D neutron generator employs a deuteron beam which impinges on a gas deuterium target to produce a neutron beam at a beam energy of ˜8.5 MeV. A d-T neutron generator uses the deuterium (2H)—tritium (3H) reaction to generate neutrons. Deuterium atoms in the accelerated beam fuse with deuterium and tritium atoms in the target to produce neutrons and alpha particles.
There is therefore a need for a compact, low-cost, high-intensity, material-specific primary or secondary inspection system and method suitable for deployment. As a secondary inspection method and system, there is a need for that method and system to clear or confirm alarms of a primary system in a relatively short inspection time with high throughput.
Therefore, what is needed is a compact, high-yield and deployable targeted neutron inspection system that results in short inspection times.